Generally, automotive body panels are formed from metal sheet using various forming methods to create the desired shape. For example, to form a roof header for a truck cab, processes such as contour roll forming and stretch forming may be used. Two separate processes are generally required because the profile of a roof header has a compound angle and a non-uniform cross section that cannot be formed by a single operation.
Contour roll forming is one process used to form metal sheet into a desired shape. Contour roll forming is a cold rolling process in which an elongated metal sheet is fed through a series of longitudinally aligned roll stations to progressively shape of sheet. Contour roll forming may be used for symmetrical and asymmetrical shapes of uniform cross section.
A typical roll forming station includes a set of roller dies which are contoured for forming the metal sheet into the desired shape. The roller dies are set opposite each other along a parallel axis, for creating the upper and lower profile of the metal sheet. The roller dies may be positioned side by side or above and below each other depending on the shape forming requirements.
Roll forming produces lengths of material having a constant crosssection; however, many articles, such as roof headers for pickup trucks, require an additional longitudinal curvature. Therefore, the roll formed stock must then be stretched in a separate operation to create a curved outer edge resulting in a non-uniform cross section. In stretch forming, the metal sheet is positioned over a form block or mating dies having the desired profile. The sheet is then held in tension and stretched beyond its yield point, causing the metal to take the desired shape. Unlike contour roll forming, stretch forming may be used to create shapes with compound curves, twists and bends in multiple planes. A bending machine including a form block having adjustable grippers on either side is generally used to form a parabolic curve along the outer edge of the roll formed stock. Alternatively, the metal sheet may be placed between upper and lower dies and the parabolic curve formed with the mating dies. Adjustable grippers on either edge of the dies hold the metal sheet in place and stretches the material between the dies.
A disadvantage of this manufacturing process is that it involves two forming steps, there by requiring a multiplicity of machinery and abundance of time. Each forming method requires its own specific machinery in addition to the auxiliary equipment required for production operation. Stretch forming using a bending machine has the further disadvantage that is slow and not well suited to high volume production. Also, stretch forming requires extra metal sheet to allow for gripping the work piece during the forming process. Additional material must also be provided because of the possibility of spring back after the stretch forming process.
Clearly, it is desirable to consolidate or eliminate the requirement of using separate roll forming and stretching operations in the manufacture of complexly contoured articles such as truck roof headers. However, it has not heretofore been possible to roll form sheet metal while simultaneously stretching the sheet metal to introduce a longitudinal curvature in the plane of the sheet. As will be detailed hereinbelow, the present invention provides a novel roll forming process which also stretches a workpiece.
SUMMARY OF THE INVENTION
According to the present invention a stretch roll forming apparatus is provided that includes a plurality of roll forming stations. The roll forming stations are serially disposed so that a sheet metal workpiece may be advanced through each of the stations in sequence. Each station includes a pair of roller dies supported by an inboard support station and an outboard station, in a spaced apart relationship, so as to each be rotatable about a horizontal axis. The roller dies are configured to shape a sheet metal workpiece which is advanced therebetween. At least one of the roll forming stations comprises a stretch roll forming station wherein the roller dies each have a frusto-conical shape. The roller dies are disposed so that the taper of one of the roller dies extends from the inboard support to the outboard support, while the taper of the other die extends from the outboard support to the inboard support. When a sheet metal workpiece is advanced between the roller dies of the stretch roll forming work station, a first longitudinal edge of the workpiece is stretched relative to a second longitudinal edge such that a longitudinal curvature is imposed on the workpiece.
In some embodiments of the present invention, one of the roll forming stations is a stretch roll forming station while the other stations are non-stretch roll forming stations. The stretch roll forming station may be the final roll forming station or one or more non-stretch roll forming stations may be positioned after the stretch roll forming station. In other embodiments of the present invention a sweep station is provided which is operative to impose a curvature on the sheet metal workpiece out its original plane. The sweep station may be provided downstream of the roll forming stations.